The invention concerns a vibration exciter with load compensation for the dynamic excitation of test specimens, comprising a base, an actuator, an armature which can be moved by means of the actuator in an excitation direction relative to the base and guided by a linear guiding means parallel to the excitation direction and a pneumatic load compensation means which compensates for the gravity force of at least the armature, preferably in addition to that of the test specimen being excited.
Vibration exciters, often also called shakers, find their use in the investigation of the dynamic properties of a test specimen in a vibration testing layout. Vibration exciters are primarily used for purposes of materials science, wherein components or entire assemblies form the test specimen, for example. For this, the test specimen is excited with a defined test signal and at the same time the position and/or the state of motion of the test specimen is determined by suitable measurement means. These measurement means can have, besides strain gage strips, also vibration sensors or other sensors for positional determination or for determining the state of motion. From these determined measurement quantities, conclusions can be drawn as to the mechanical behavior, such as the fatigue behavior, for example.
Moreover, vibration exciters are known which are suited to investigating the actual measurement means, such as vibration sensors, for the investigation of vibrational processes, but also especially calibrating them, as proposed in ISO 16063.
The diversity of known vibration sensors in terms of measurement range, design size, and so forth, is very large. Besides very small and light vibration sensors for accelerations of up to one million g, where 1 g corresponds to the mean acceleration of the earth of 9.81 m/s2, very large and heavy vibration sensors are also known, such as are used for example in seismometers.
Seismometers are used to detect ground tremors of earthquakes and other seismic waves. For this, seismometers have a mass mounted to vibrate on springs, and its mechanical vibration is measured. To determine the direction of an occurring ground tremor, seismometers are known with up to three such vibrational spring and mass systems. The mechanical vibrations such as typically occur during seismic processes are very small; the smallest detectable accelerations lie in the range of a few billionths of the normal mean acceleration of the earth, or g. The frequency range extends from around 50 Hz to only around one vibration per hour. The seismic mass needed for this, being 10 kg or more, is therefore very large.
Owing to the very small mechanical vibrations which occur, very high demands are placed on the mechanical boundary conditions of the vibration testing layout for the calibration of such large vibration sensor, i.e., vibration sensors with large seismic masses. With the vibration exciter it should be possible to generate a high-quality sinusoidal and perturbation-free exciter signal, distinguished by the most exactly maintained vibration amplitude, a low (<5%) distortion factor, i.e., the degree of unwanted distortions of the exciter signal, as well as a steady signal variation.
For the calibration of vibration sensors with a horizontal alternating vibration excitation, it is known how to use air bearings to reduce the friction. Especially high precision and dynamics of motion can be realized as compared to conventional mounting by plain bearings, roller bearings, or spring bearings. The basic notion of the air bearing consists in that the object being supported, as it were, floats on a film of air, such that the object being supported runs free of friction, except for the very low fluid friction inside the film of air and in the boundary layers of the air film with the adjoining surfaces. Accordingly, air bearings enable a low-friction movement of an object. Basically both linear and rotary air bearings are known, which in turn can be divided into static as well as dynamic air bearings. The geometrical appearance of air bearings is diversified. For example, cylindrical, rectangular or planar forms, as well as special geometrical forms are known.
In the case of the calibration of vibration sensors with a vertical alternating vibration excitation, the force needed for the vibration excitation is superimposed on the static gravity force of the vibration sensor in the earth's gravity field. In order for the same magnitude of the force of vibration excitation to be available for both excitation directions, i.e., contrary to and concurrent with the gravity force of the vibration sensor, it is known how to compensate for the weight forces. Various methods are known for this, what is common to all methods being that an additional static force is exerted, contrary to the gravity force of the test specimen being excited, such as a vibration sensor. The forces needed for this load compensation can be generated either electrodynamically, hydraulically, pneumatically, or by a spring force.
A calibration device, CS18 VLF, is known from the firm Spektra Schwingungstechnik and Akustik GmbH for the calibration of acceleration sensors in the horizontal and vertical direction with a vibration exciter. The vibration exciter comprises an actuator designed as a linear drive, which is used for the dynamic excitation of masses in the excitation direction. The movable part of the calibration device, a linear movable carriage, also called the armature, is connected by means of a linear designed air bearing to the stationary part of the calibration device, a base. For the calibration of vibration sensors in the vertical excitation direction, the calibration device comprises furthermore an electrical zero position control, which compensates for the gravity force exerted by the carriage and the vibration sensor. For this, a portion of the driving energy of the mentioned linear drive is used, so that the full force available for the actual vibration excitation does not have to be used against the gravity force.
Moreover, load compensations are known in which mechanical spring forces are used in order to compensate for the gravity force of a test specimen being excited. Basically, the drawback in the use of steel springs are the given strength limits and the resilience, as well as the possible excitation of resonances. When using elastomer springs, in addition nonlinearities arise which need to be compensated in order to generate a low-distortion exciter signal. Furthermore, the mentioned springs cause discontinuities in the exciter signal due to the external and internal friction present, such as are manifested for example in the stick-slip effect (static friction effect).
Moreover, load compensations are known which have a compressed medium, and thus one forming an excess pressure.
On the one hand, hydraulic cylinders are known for load compensation, such as are used for example to study the vibration behavior of entire vehicles. The compressed medium, in the present case hydraulic fluid, is kept in a closed system, while certain parts, in the present case the piston rod of the hydraulic cylinder, extend beyond the boundaries of the system. Accordingly, seals must be provided, such as shaft O-rings. These seals on the one hand have a disadvantageous wearing behavior and furthermore cause discontinuities in the exciter signal being generated due to the friction occurring.
Furthermore, it is known how to use compressed air in a rubber bladder load compensation. Thus, Tira GmbH offers a vibration testing layout which uses a so filled rubber bladder for the load compensation. The rubber bladder here is arranged beneath the moving part, the armature, in order to compensate for its gravity force. Frictional forces occur between the rubber bladder and adjoining components of the vibration testing layout, which in turn cause discontinuities in the exciter signal being generated.
The drawback to all known possibilities of load compensation is therefore that additional perturbation signals are generated by the friction and other nonlinearities occurring, which prevent the generating of a purely sinusoidal signal form, such as is needed for the calibration of vibration sensors, especially vibration sensors of large mass, i.e., in the range of 10 kg or more.
One problem which the invention proposes to solve is therefore to provide a vibration exciter with load compensation for the dynamic excitation of test specimens, wherein the friction and other nonlinearities occurring during the load compensation are minimized in order to generate high-quality low-perturbation exciter signals and wherein both the vibration path and the force needed for the vibration excitation are available in both excitation directions, i.e., directed contrary to and in the same direction as the gravity force, independently and unlimited by the load compensation of the gravity force.